1. Field of Invention
The present invention relates to a temperature sensor. More particularly, the present invention relates to a temperature sensor which having two capacitors coupled to temperature-independent voltage sources for increasing S/N ratio of the temperature sensor.
2. Description of Related Art
Recently, bipolar transistors or diodes are used as temperature sensors in CMOS technique. FIG. 1A shows a conventional temperature sensor. In the temperature sensor 10, the diode is selectively connected to two current sources 130 and 132 by using a switch 102. During operation, the diode 100 is connected to the current source 130 to receive the current If1. The voltage difference ΔV between capacitors 120 and 122, each of which has a capacitance C1, can be obtained by the following equation:
      Δ    ⁢                  ⁢    V    =            kT      q        =          ln      ⁡              (                              If            1                    /                      I            s                          )            
Accordingly, after connecting diode 100 to the current source 130, the diode 100 is connected to the current source 132. Thus, after operations of switch capacitor circuits, which includes capacitors 120˜126 and the comparator 110, the output voltage Vout of the temperature sensor 10 could be obtained by the following equation:
  Vout  =                    C        ⁢                                  ⁢        1                    C        ⁢                                  ⁢        2              ⁢          kT      q        ⁢    ln    ⁢                  If        1                    If        2            
In other words, the output voltage Vout is in proportion to the absolute temperature k. However, it is rarely to operate at circumstances wherein the temperature is lower than −40° C. Therefore, although the temperature sensor has a theoretically sensing range between −273° C. and, for example, 127° C., the sensing range between −273° C. and −40° C. is rarely used. Referring to FIG. 1B, a voltage-temperature curve diagram of a common-used temperature sensor is shown. As shown in the Figure, the output voltage of the temperature sensor is 3.3V when the temperature is 127° C., and the output voltage is 1.9V when the temperature is −40° C. It is obvious that the voltage range between 0V and 1.9V is wasted when a part of sensing range between −273° C. and −40° C. is not used. Accordingly, only part of the sensing range (1.9V˜3.3V) is used to represent a temperature range −40˜127, and therefore precision of the result cannot be increased due to the limitation.